The present invention relates generally to methods and apparatus for supplying pressurized fluids. More particularly, the present invention relates to methods and apparatus for pumping fluids into a wellbore at a wide range of pressures and flow rates.
The construction and servicing of subterranean wells often involves pumping fluids into the well for a variety of reasons. For example, fluids may be pumped into a well in conjunction with activities including fracturing, completion, stimulation, remediation, cementing, workover, and testing operations. A variety of fluids used in these operations include fracturing fluids, gels, drilling mud, barite, cement, slurries, acids, and liquid CO2. In each of these different applications, the fluid may be required to be pumped into the well at any point within a wide range of pressures and flow rates.
Pumping units often utilize a power source, such as a diesel or electric motor, to drive one or more pumps. Many pumping units utilize a multispeed transmission connected between the power source and the pumps. The transmission operates to expand the speed and torque range produced by the power source by providing a set number of gears that transfer the motion produced by the power source to the pump.
Most multispeed transmissions provide a broad operating envelope of speed and torque within which a pump can operate. This operating envelope 10 can be illustrated as a relationship between pressure and flow rate as is shown in FIG. 1. Line 15 defines the peak hydraulic horsepower at which the pump can operate and line 16 defines the peak torque output. Because the transmission comprises a limited set of gear ratios 17, the operating envelope 10 of the pump has discreet points 20 at which the pump can operate at peak hydraulic horsepower. These discreet points 20, in effect, create gaps 25 where the pump cannot operate with a given gearing.
Although gaps 25 can be reduced by increasing the numbers of gear ratios within a transmission, as the number of gear ratios increases so does the complexity and weight of the transmission. Therefore, there are often practical limits on the number of gear ratios at which a transmission can operate. Thus, there remains a need to develop methods and apparatus for pumping fluids into a wellbore at wide range of pressures and flow rates, which overcome some of the foregoing difficulties while providing more advantageous overall results.